Most women wear bras that don't fit properly. This is largely due to the fact that many of them have never correctly measured their breast size. Once you know your accurate bra size, you'll find that bras fit much more comfortably and provide more support. Here's an easy way to measure your breast size.
Stand up straight, with both feet planted firmly on the floor.
Take a tape measure and wrap it around your chest, directly along the line where your bra band would be. The tape measure should be directly underneath your breasts. Your arms should be down and the tape measure should be snug against your body. Round the number on the tape measure to the nearest even number. If you measured 34.5", you would write down 34". If you measured 39.5", you would round up to 40".
Wrap the tape measure around your chest, at the fullest part of your breasts. The tape measure should be resting loosely on the fullest area of your breast. Do not pull the tape measure too tight, or you will flatten your breasts and get an inaccurate measurement.
Determine your cup size. To do so, take the band measurement and subtract it from the breast measurement. If the difference is less than 1", you are an AA cup. If it is 1", you are an A cup. If the difference is 2", you are a B cup; 3", a C cup, and so on.
Write down these measurements so that you can refer to them when bra shopping.
Consult a professional at a lingerie store if you have difficulty determining your bra size. She will be able to help you with measurements and give you a correct reading.